A little while ago I took Brian Masney's pam_quota and did a quick hack to get it working with XFS for my college networking society (netsoc-dkit).
Then, I posted it to my public_html directory and prompty forgot about it- Until Now!
So if you need pam_quota for xfs download pam_quota_xfs.c,
build instructions are included in the file.
P.S. Even though it has been in constant production use on a busy Gentoo server without issue, be careful as I haven't tried to break it!